Moves TabsView into jck-library-extensions.

Squashed commit of the following:

commit 927d4f1a23b4b15709358ed8007f868c7c823748
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Sat Jun 7 17:15:51 2014 -0500

    Moves TabsView to jck-library-extensions.

commit 8771460347583d3203050d5a5eca47d752904ba5
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Sat Jun 7 16:43:44 2014 -0500

    Refactors tabs.js, adds jck-library-extensions.